A standard drying apparatus comprises a housing, a conveyor for displacing flat workpieces horizontally in a travel direction through the housing on respective vertically offset levels, a respective horizontal array of horizontally elongated upper nozzle boxes lying generally in a plane above each level and a respective horizontal array of horizontally elongated lower nozzle boxes lying generally in a plane below each level. Each lower box is formed with a plurality of upwardly directed nozzle holes and each upper box is formed with a plurality of downwardly directed nozzle holes. Heated air is fed to the boxes and projecting the heated air from the holes against the workpiece for drying same. Such a system is used, for instance, to dry freshly pressed gypsum-board panels to cure the binder therein and stiffen same. The air is recirculated inside the dryer as described in U.S. Pat. No. 5,351,416 of Witkin and German patent 843,386 of Krantz to avoid polluting the surroundings with any particles or gases picked up.
It is further known, for example from German utility model 1,871,618, German 1,196,516 of Sievers, German 1,460,672 of Mohring, German 1,596,520 of Carson (U.S. priority 604,006), German 2,029,494 of Hermann, German 4,412,071 of Honcamp, and German 4,331,496 of Baum to provide respective upper and lower shield plates each formed with a plurality of apertures and each slidable on a respective one of the boxes between a position with the apertures aligned with at least some of the respective holes and a position with the apertures out of line with the respective holes. This makes it possible to block some of the holes of the nozzle boxes to reduce the air flow therefrom when narrow workpieces, that is workpieces not wide enough to fill the press, are conveyed through the apparatus. This prevents heating capacity from being wasted.
The problem with these systems is that the changeover between wide and narrow workpieces is a laborious operation. The various shields must be shifted, normally one at a time, into the desired position. This not only entails opening the housing to gain access to the shield plates, but also requires each plate to be unclamped, shifted, and reclamped before the housing is closed up again. Changing workpieces on the fly is of course impossible, and in fact it is necessary to shut down the whole production line for a while on shifting between wide and narrow work pieces.